I am in the process of restoring an American Bosch Model 450, in which someone hacked out the on/off switch+volume control. I have a new one installed, and was wondering whether it mattered where the three wires that go to the volume control get soldered to. Does a certain wire have to be on a specific pin, or can I just wire them up any way I want?

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Last edited by 5Y4GT on Dec Mon 04, 2017 11:27 pm, edited 3 times in total.

Hi, look at the schematic and pictorial diagrams attached. The pictorial shows the volume control mounted with the three terminals showing as when the chassis is turned over for service. The center terminal goes to the .005 mFd. cap No. 56 which is on the central board with all of the components. This center terminal is the rotor of the control, that travels from one end of the circular resistance element inside the control from one end terminal to the other, picking up higher or lower volume. The right hand or low side terminal (when looking at the pictorial, which assumes you have the controls of the chassis facing you) connects to the cathode of the 75 tube. The left hand terminal, the high side, connects to the IF transformer. If you got the right and left terminals reversed, the control would operate backwards and without proper gradation of volume, at that. Notice that the wires from the center and high side are twisted together; this is to reduce hum pickup.

Note: you definitely don't want to get the power switch wires, that go on the back of the control, mixed up with these volume wires!

Its connected there, but the other side of it (resistor 54) is disconnected. Someone cut the on/off switch-volume control out of the radio . I was wondering if this wire went to the volume control as well.

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Do you mean the other side of the IF can is connected to the high side of the volume control as well, because I have four wires (three are connected) and three pins on the switch. The only wire not connected is the one that comes off of resistor 53. Does this wire and the wire from the other side of the IF can connect to the high side of the volume control? Someone drew up the pictorial diagram a little different than how the radio was made.

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I also have a question on how to wire the power switch for this radio. So far, I have wired it so that one side of the plug goes to one side of the primary winding of the transformer, and then to the switch (see pictorial diagram and/or schematic). Now, I have the other side of the primary winding and one side of the power cord loose, and one terminal free on the switch. I'd think they go there if it weren't for the .01 mfd cap that is supposed to ground off of one side of the PT's primary winding (# 72 on schematic) and that if the radio was wired that way, it would be on the moment it was plugged in. I just want to know how the other side of the switch is supposed to be wired so I can wire it and power up the radio soon.

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Remove the wiring you have done and proceed as follows: One side of AC line to one side of transformer primary. Other side of AC line to one switch terminal. Other side of transformer primary to the remaining switch terminal: I would connect capacitor 72 here also, and then the other side of capacitor 72 to ground. This disconnects the cap from line when the set is off, which is better than the way the schematic shows it. This capacitor should be a safety cap, available from various sources including Sal, yellow ad over on the right.

Thank you! Will do it that way now. I should be able to power it up soon and hope that my repairs were successful. I will probably get a safety cap, but is it definitely necessary if I already have a fuse on the hot side of the power cord (I used a modern polarized cord)?

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The safety cap is designed to fail open, if it ever fails. Other types of caps could possibly short if they fail, connecting line voltage to the chassis and thus causing a shock hazard. You could use a regular 630 volt cap here temporarily but order a safety cap and install ASAP. I keep several of each type, for use on transformer-operated or AC/DC radios, in stock. Safety caps weren't available when the set was built, but are now, so best to use them.

(Actually what I said in the previous post about the cap being removed from line voltage when the set is turned off is not entirely true: it would still be connected to the line through the transformer primary. However, there is no way to not have it connected to the live line no matter where you would put it, so let's leave it as I first stated.)

Well, I guess I have bigger fish to fry. I tested the speaker again for this set and...yep, the field coil is open. I'm not too happy, since the speaker cone is in crazy good shape. Well, now I know why someone used this radio for parts.

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Does anyone have any suggestions on how to rewind the field coil? So far, this is my procedure:

1. Remove speaker cone and voice coil2. Remove screws connecting field coil frame to speaker basket3. Remove field coil4. Unwind field coil, testing at various points to make sure that it definitely is open5. Rewind field coil if break is found, or if all wire is removed6. Reassemble speaker7. Power up radio

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I would remove all the old wire and rewind with new. For every actual break, there is a least one place that **almost** broke. This obviously depends on some other details, but why go to all that work, and then leave the old wire in there?

Note also that the insulation quality on modern wire might be better than the original.

Anyways...I've removed the field coil, and was able to salvage the original speaker cone. Should I build a simple coil-winding jig that uses a jig? Also, how should I obtain a micrometer? I have one, but it is frozen in place and is useless at this point?

_________________No matter how smart you are, there is always someone smarter than you.